132 HUMAN PHYSIOLOGY. 



The complemental air is that amount which can be taken into the lungs 

 by a forced inspiration, in addition to the ordinary tidal volume, and amounts 

 to about no cubic inches. 



The reserve air is that which usually remains in the chest after the ordinary 

 efforts of expiration, but which can be expelled by forcible expiration. The 

 volume of reserve air is about 100 cubic inches. 



The residual air is that portion which remains in the chest and cannot 

 be expelled after the most forcible expiratory efforts, and which amounts, 

 according to Dr. Hutchinson, to about 100 cubic inches. 



The vital capacity of the chest indicates the amount of air that can be 

 forcibly expelled from the lungs after the deepest possible inspiration, and 

 is an index of an individual's power of breathing in disease and during pro- 

 longed severe exercise. The combined amount of the tidal, the comple- 

 mental, and the reserve air, 230 cubic inches, represents the vital capacity 

 of an individual five feet seven inches in height. The vital capacity varies 

 chiefly with stature. It is increased eight cubic inches for every inch in 

 height above this standard, and diminishes eight cubic inches for each inch 

 below it. 



The tidal volume of air is carried only inta the trachea and large bronchial 

 tubes by the inspiratory movements. It reaches the deeper portions of the 

 lungs in obedience to the law of diffusion of gases, which is inversely pro- 

 portionate to the square root of their densities. 



The ciliary action of the columnar cells lining the broncial tubes also 

 assists in the interchange of air and carbonic acid. 



The entire -volume of air passing in and out of the thorax in twenty-four 

 hours is subject to great variation, but can be readily estimated from the 

 tidal volume and the number of respirations a minute. Assuming that an 

 individual takes into the chest twenty cubic inches at each inspiration, and 

 breathes eighteen times a minute, in twenty-four hours there would pass in 

 and out of the lungs 518,400 cubic inches, or 300 cubic feet. 



Chemistry of Respiration. As the inspired air undergoes a change in 

 composition during its stay in the lungs which renders it unfit for further 

 respiration, it becomes requisite, for the correct understanding of respiration, 

 to ascertain the composition of both inspired and expired air. 



Composition of Air. Chemic analysis has shown that every 100 volumes 

 of air contain 20.81 volumes of oxygen, 70.19 volumes of nitrogen, and 0.03 

 volume of carbonic acid. Aqueous vapor is also present, though the quan- 

 tity is variable. The higher the temperature, the greater the amount. 



